Many techniques and devices have been recently developed in the field of optics for measuring the specular reflectance of a particular sample. When deciding which of these techniques or devices would be the most appropriate for a particular surface, several factors such as the approximate value of the reflectance, whether this reflectance is to be measured in the visible, infrared or vacuum ultra-violet (VUV) spectrum, and what level of accuracy is desired must be considered.
One such technique is discussed in the Journal of the Optical Society of America, Volume 50, No. 1, Page 1, January, 1960 by H. E. Bennett and W. F. Koehler and in U.S. Pat. No. 3,499,716 to Bennett. This method of measuring reflectance utilizes the so-called V-W or Strong-type reflectometer. This device uses a revolving mirror and a movable sample so that the source beam first strikes the mirror in a reference position to obtain a hundred percent signal and then the mirror is moved to a reflectance position and the sample is inserted in the light beam in such a manner to allow the beam to be twice reflected off the sample, providing a measurement of the square of reflectance. While the optical path length of the beam of light is unchanged, it should be noted that to reduce problems of detector surface sensitivity variations with misalignments, an integrating sphere is placed in front of the detector so that its surface is always uniformly illuminated even with sample misalignment. However, one disadvantage of this approach is the limited wavelength usefulness of the Bennett device in conjunction with these integrating spheres, thereby allowing reflectance measurements to be made only in the visible and infrared regions. This is due to the excessive number of reflectances which must take place and the use of the integrating sphere. Furthermore, the V-W measurement technique does not lend itself toward taking measurements of transient reflectance effects due to the relative long time duration between the measurement of the reference and reflectance signals.
An apparatus for the measurement of vacuum ultra-violet (greater than 500 A) optical properties is reported by Madden and Canfield in the Journal of the Optical Society of America, Volume 51, No. 8, 1961, Page 838. This device employs the single beam reflectance measurement technique of utilizing a movable detector and sample. To obtain the reference reading, the sample is removed from its position and the detector is rotated to a position for measuring the unperturbed beam. Of special importance is the fact that this technique permits measurement of the angular dependance of reflection and polarization effects (with the addition of a polarizer at the source). Furthermore, the optical path length is identical between the detector and source at both reflectance and reference positions; therefore, source beam divergence or intensity is the same on the detector at both positions. In addition, this technique can be used over a broad range of wavelengths from the X-ray to far infra-red, limited only by source and detector problems. However, one such problem area with this technique is the positioning of the detector accurately so that the source beam falls on the same portion of the detector surface at all times. This is an acute problem since the detector is in motion and a 1.degree. misalignment of the mirror in angular positioning would represent twice the area in reflectance measurements at the detector. A further problem relates to the stability of the light source. Since several minutes, if not longer, will elapse between reflectance and reference measurements, a shift in the source output will directly affect the signal, causing an error in the reflectance readings.
A classical solution to several of the problems associated with the single beam technique is to utilize what is called the dual beam reflectance measuring technique. This method is identical to the single beam procedure with the addition of an oscillating mirror and a detector reference. In this particular method, the oscillating mirror focuses the light from a source or monochrometer, first onto the detector reference, and then onto the sample or first detector. This system provides a constant monitoring of the light source output, permitting corrections for drift of the source between the reflectance and the reference signal measurements with the first detector. However, problems arise in alignment of the oscillating mirror and the sample as mentioned in the single beam measurements. Additionally, if the reference detects drifts or changes in sensitivity, this will be interpreted as a change in light source output, and an error in reflectance will occur.
Additionally, several reflectometers utilize a reference sample as a reference reading for comparison to the reflectance reading. Since these samples are subjected to the perturbing atmosphere, they can be contaminated thereby causing an error in the true reflectance values.
A review of the prior art devices, therefore, reveals that no device has been developed which both operates in the infra-red, visible, and vacuum ultra-violet regions and also contains a fixed source and detector having identical path lengths, thus eliminating misalignment errors. Furthermore, none of the references is capable of measuring fairly rapid changes in the reflectance of a sample which is exposed continuously to a perturbing environment.